A social media contest is a promotion in which pre-defined performance or skills determine who wins. For an ordinary Belgian business, "like and we'll draw a winner" is not a safe default: VLAIO warns about the lottery ban. Free participation does not automatically make a random draw permitted.
Before publishing, determine how participants will be assessed, which prize is available and how you will use their data. Also check the rules of Facebook or Instagram. Below is a workable preparation checklist with a fictitious example; it is not a complete legal assessment of your promotion.